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In this answer I would like to understand

What does it mean in Node Editor that light will interact with volume?

I would like to see a few pictures showing the difference/meaning.

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Some shaders determine what happens when the light is reflected or emitted by the surface of an object.

But what happens to the light when it goes through or inside the object, is determined by the volume

In blender volume can be used for objects or for the world.

enter image description here enter image description here

There are many uses for volume shaders, like creating smoke or a hazy atmosphere and volumetric lighting or to simulate liquids or glass.

The main concepts to understand are Scattering, Density, Anisotropy and Absorption.

Scattering

The reason we see rays of lights on a dusty, or hazy environment is because there are suspended particles on the air that reflect the light.

Photons are scattered when they collide with particles. When light travels through a volume in a certain direction, photons are deflected in other directions when they collide with particles. This phenomenon is called scattering. How frequently light is scattered by particles both depends of the particles properties (mainly their size) and their density in the volume (how many particles/molecules are in one unit of volume)

(https://www.scratchapixel.com/lessons/procedural-generation-virtual-worlds/simulating-sky/simulating-colors-of-the-sky)

Density

Determines how thick the volume appears to be. More particles (or a denser environment) will result in less light travelling through, and more light being scattered .

Anisotropy

Controls the look of the scatter effect depending on the direction of the light passing through it.

Positive values let the ray scatter in the direction it is traveling, while negative values cause the ray to scatter opposite of its travel direction.

![enter image description here

For positive values, rays mostly get scattered along the direction of the ray that entered the volume. Negative values mostly scatter into the opposite direction. Anisotropy of 0.0 (center) will result in random scatter directions. Depending on the density, rays might not get scattered at all.

(from: https://www.blenderdiplom.com/en/tutorials/all-tutorials/582-cycles-volume-scatter.html)

Absorption

Light is absorbed as it passes through the volume, a black liquid like black ink will absorb all of the light passing through it, for example. A glass or liquid can act as a color filter by absorbing some colors and letting others go through unfiltered.

enter image description here

How to make glass clear at overlapping point


Practical examples:

Light passing through an object with Volume scatter (suspended particles, like smoke, haze, dust, smoke)

Atmospheric lighting in cycles

enter image description here

or light being emitted inside a volume

enter image description here

Fire and smoke:

enter image description here

enter image description here

How to create fire materials using the new Principled Volume node that visually look the same as the "old method" of fire materials?

Volume absorption makes the object transparent but absorbs light which passes through it. It can be used whenever you need a material which absorbs light, such as water or colored glass.

Here's an example, the liquid has no color, but the volume absorption node makes the light that goes through it be red:

enter image description here

enter image description here

fluid in a glass


Further reading:

https://www.scratchapixel.com/lessons/advanced-rendering/volume-rendering-for-artists

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  • $\begingroup$ Mr susu, I just love how well written your answers are :). $\endgroup$ – Jachym Michal Sep 8 at 22:47
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    $\begingroup$ Yup! this is one of those that will be a go-to reference.. $\endgroup$ – Robin Betts Sep 10 at 18:33

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